As a rotor used in a rotary electric machine, there has been known a rotor in which a plurality of permanent magnets are arranged in a rotor core at predetermined intervals in a circumferential direction (for example, see JP-B-5278003, JP-A-2002-78260 and WO2012/014836).
A rotor for an electric motor described in JP-B-5278003 reduces torque ripples and iron loss by arranging a position of a groove, which is provided in an outer circumferential portion of the rotor, at an electric angle of 40° to 44° with respect to a center line of a magnetic pole closest to the groove. Alternatively, the electric angle is set to 44° to 53° to reduce harmonic components in a no-load induced voltage. A rotor for a permanent magnet motor described in JP-A-2002-78260 prevents a rise in peak value caused by harmonic components included in an induced voltage to enable high-speed rotation by setting an angle between two outer circumferential grooves provided in an outer circumferential portion of a rotor core to be equal to an angle between two magnetic poles at a stator side. A rotor described in WO2012/014836 reduces cogging torque and torque ripples by reducing an outer diameter of a rotor core as approaching to both circumferential ends of a pole pitch angle from a center of a magnetic pole.
Since the torque ripple is a major factor of annular vibration of the rotor, vibration and noise of the rotor can be further suppressed if the torque ripple can be reduced. The torque ripple can be reduced by the techniques described in JP-B-5278003, JP-A-2002-78260 and WO2012/014836. However, it is desired to further reduce the torque ripple in order to improve silence of a vehicle which includes a rotary electric machine mounted thereon.